Digitized dark-field scanning transmission electron microscope images were acquired using a PC-AT computer system. The computer was interfaced to the Vacuum Generators HB501 STEM and used to generate a rapid scan of the nanometer size electron probe (approximately 50 microseconds per pixel) . The annular dark-field signal was measured by counting single elastically scattered electrons at each pixel. Images were analyzed on a Macintosh II computer, allowing quantitative measurements of masses of macromolecules to be made. Initial tests were performed on freeze-dried T4 bacteriophages.